This invention is directed to electrode configurations in a liquid-metal plasma valve for minimization of offswitching time and maximization of offswitch current and voltage holdoff.
The broad field in which this invention resides is the liquid-metal cathode electrical conducting device field which includes mercury arc rectifiers and the like. The typical application of a liquid-metal cathode device for converter valve service, either for rectification or inversion, is characterized by long-term continuous operation at relatively high average current and relatively low peak current. Modern day voltage holdoff requirements go up to about 500 kilovolts. Under these conditions, it is generally preferable to employ a three-element valve such as is shown in W. O. Eckhardt U.S. Pat. No. 3,659,132 wherein the condenser is at cathode potential. This condenser reduces the background pressure of the liquid-metal vapor and collects neutral atoms and ions at offswitching to permit a rapid increase in holdoff voltage to a high value. In such valves, it is desirable to have the condenser at cathode potential because liquid-metal recirculation is necessary, and isolators for the recirculation line become difficult or impractical at high voltage holdoff levels.
DC switching applications have somewhat different requirements from the converter application. In any case the current rating determines the size of the jet of ionized liquid metal and hence the size of the electrodes while the positioning and shape of the cathode, anode and condenser control offswitching time.